The present invention relates generally to investigating earth formations traversed by a borehole. More particularly, the invention relates to methods for determining values for and for further characterizing the attributes of a formation surrounding a borehole by processing well logging data derived by lowering in the borehole one or more apparatus for investigating subsurface earth formations. The ability to assign values for (e.g. quantify) and further characterize formation attributes (e.g. formation minerals) permits a wide range of new and improved results to be obtained through logging, including a direct calculation of cation exchange capacity (CEC) and a corrected water saturation (Sw) determination, an oil API gravity log, an improved grain density and porosity determination, an improved understanding of depositional environment, a permeability log, a log of mean grain size, a log of thermal neutron capture cross-section and a log of formation elements not directly measurable, among others.
Quantitative knowledge of the lithological constituents present in a well as a function of depth would be valuable in assessing all aspects of exploration, evaluation, production, and completion. A complete shaly sands lithological description must go beyond simple discrimination between "sands" and "shales" and, for example, establish the quantity of clay minerals in all layers including so-called "clean sands", identify and quantify the non-clay as well as clay minerals present, and identify subtle and pronounced changes in depositional or diagenetic facies by characterizing the formation minerals.
Until now, it has been generally accepted that there are no successful techniques available for taking elemental chemical data and deriving therefrom a quantitative mineralogical analysis of the lithology in question. Some procedures have been posed for gaining limited knowledge of lithology from chemical data, and different logging tools have been utilized to provide elemental data and indicators, but none of the previously known procedures or tools, alone, or together, have been capable of broadly and accurately permitting quantitative mineralogical analysis from logging. The known techniques and procedures generally only address the derivation of particular outputs such as water saturation, porosity, carbon/oxygen ratios, cation exchange capacities, general lithology classifications, etc.
Examples of borehole tools which provide and determine elemental chemical data and yields include natural gamma ray tools, induced gamma spectroscopy tools and high resolution spectroscopy tools. The natural gamma ray tools typically comprise a scintillator and pulse height analyzer which respond to and measure the gamma ray activity due to the decay in an earth formation of the naturally radioactive elements: thorium, uranium and potassium. In the past, the thorium plus potassium content has been used as an indication of clay or shale content. Uranium amounts have been suggested to indicate organic carbon sources and to provide information regarding secondary porosity detection and fractures. See e.g. U.S. Pat. No. 4,071,755 issued to Supernaw et al. As detailed by Lock, G. A. and Hoyer, W. A., "Natural Gamma-Ray Spectral Logging," The Log Analyst, September-October 1971, pp. 3-9, a thorium/uranium ratio may in some instances provide insight into the type of marine environment encountered. A potassium percentage determination may provide in some instances an indication of potash deposits or micaceous sands.
Induced gamma ray spectroscopy tools typically utilize a pulsed deuterium-tritium accelerator neutron source and sodium iodide detectors, which detect the gamma rays resulting from the interaction of the source neutrons with the formation elements. As disclosed in U.S. Pat. No. 3,521,064 issued July 21, 1970 to Moran and U.S. Pat. No. 4,055,763 to Antkiw, the spectroscopy tools can be run either in an inelastic or an activation mode and provide elemental yield information on hydrogen, chlorine, silicon, calcium, iron, oxygen, carbon and sulfur. Using various ratios of the determined elements, indicators such as fluid salinity, porosity, shaliness, lithology and oxygen activation, among others, may be determined.
High resolution spectroscopy tools are based on the same principles as the induced gamma ray spectroscopy tools except that the accelerator neutron source may be replaced, if desired, by a chemical source, and the detectors utilized are high resolution (such as high-purity germanium) detectors. The high resolution (or enhanced resolution) spectroscopy tools (see Everett, R., Herron, M. and Pirie, G., "Log Responses and Core Evaluation Case Study Technique Field and Laboratory Procedures" SPWLA 25th Annual Logging symposium, June 27-30, 1983 pp. 23-24), may be used to determine both the amounts of the more abundant formation elements such as those determined by the induced gamma ray spectroscopy tools, and the amounts of less abundant elements such as aluminum, vanadium, magnesium, sodium, etc.
From the information gathered by the tools disclosed above, as well as other tools known in the art including electrical resistivity tools, sonic exploration tools, and other nuclear tools such as the gamma-gamma (formation density tool), or neutron-neutron (neutron porosity tool) tools, many attempts have been made to comprehensively evaluate and interpret lithology, including systems for two-mineral interpretation and shaly sands interpretation. Some systems such as SARABAND and CORIBAND (registered trademarks of Schlumberger Technology Corporation, described respectively in Poupon, A. et al., "Log Analysis in Formations with Complex Lithologies", J. Pet. Tech. (August 1971) pp. 995-1005 and Poupon, A. et al. "Log Analysis of Sand-Shale Sequences--A Systematic Approach" J. Pet. Tech. (July, 1970), correct porosity and resistivity logs for borehole and mudcake effects and then correct for the influence of clay, and/or shale content, and the effects of light hydrocarbons, etc. before computing porosity, matrix density, water saturation, movable hydrocarbon saturation, etc. Other techniques for shaly sand interpretation include the Waxman-Smits approach in which clay conductivity is used for a determination of water saturation. Clay conductivity is expressed in terms of cation exchange capacity (CEC), per unit volume, Q.sub.v. However, as shown in Burck, Lockhart, J. S., "A Review of Log and Core Methods for Determining Cation Exchange Capacity/Q.sub.v ", Transactions of the Eighth European Formation Evaluation Symposium (London, England Mar. 14-15, 1983), unless constant minerology and salinity are assumed, conventional logging cannot provide a satisfactory determination of Q.sub.v. Moreover, the Waxman-Smits approach cannot be said to provide a comprehensive evaluation and interpretation of lithology.
Another approach to lithology evaluation has been to analayze formations through core analysis. Thus, core analysis has been used to determine CEC or Q.sub.v. A summary of the different core measurement techniques is provided in the aforementioned Burck article including both destructive (pulverizing) and nondestructive techniques. In addition, core analysis has been utilized in conjunction with logging to correlate radioactive elements to cation exchange capacity. In U.S. Pat. No. 4,263,509 issued on Apr. 21, 1981 to Fertl et al., it was suggested that the cation exchange capacity determined by the laboratory testing of a cored borehole could be correlated to a function of the natural gamma rays detected by logging the said borehole. Natural gamma ray logging operations in subsequent boreholes within the same geological region would then provide, in conjunction with the predetermined function, an in situ estimation of the depth related cation exchange capacity of the subsequent borehole. Such a technique is of limited utility, however, because cation exchange capacity is being correlated to elements which generally have little global relation to the clay minerals which dictate cation exchange capacity.
Core analysis has also been used by geochemists in the analysis of depositional environments. One analysis technique is called "factor analysis" and is extensively described in Joreskog, K. G., Klovan, J. E. and Reymont, R. A., Geological Factor Analysis, Elsevier Scientific Publishing Company (Amsterdam, the Netherlands 1976). Factor analysis is a technique which can be used in geochemistry to take multiple data sets of variables such as elemental concentrations and to correlate and anticorrelate the variables such that the subject rock or formation can be described with a good degree of certainty by a small number of independent factors which can be identified. Factor analysis has been used in the past to correlate elements to desired outputs such as aerosol sources and air pollution. Thus, the detection of an increase in the abundance of the element lead would indicate increased local usage of fossil fuels. In such a correlation, score analysis is utilized to determine how the magnitude of the factors changes from sample to sample.
Factor analysis was used in conjunction with regression analysis in Tardy, Yves, Element Partition Ratios in Some Sedimentary Environments, Sci. Geol. Bull. 28, 1, p. 59-95 (Strasbourg, 1975), to classify a formation and to solve for the distribution of trace elements among the classified fractions of a rock. Thus for example, in a particular core sample set, by factor analysis, forty variables were correlated such that four groups (rock fractions) were identified: detrital, sulfide, phosphate (apatite) and organic carbon. Through the use of regression analysis, the distribution in ppm of the trace elements among the four groups was determined. Also, by analyzing results from twenty-one other sets of shale and sandstone core samples, a study of the occurrence of trace elements in identified principal rock fractions was accomplished with the resulting conclusions that environmental conditions such as weathering, deposition and diagenesis might be determinable from a determination of trace elements in the rock formation.
While the interpretation of logging results and of core data have provided many useful outputs to help describe and evaluate lithology, no techniques have been provided which can permit a comprehensive and accurate analysis of a formation by determining from initial log inputs the values for or quantities of the formation attributes such as formation minerals.
It is therefore an object of the invention to provide methods for taking log data as input and providing values for the attributes of the formation under investigation.
It is a further object of the invention to provide methods for taking log data as input and providing a mineralogical analysis including both a quantitative determination and a characterization of the minerals in the formation under investigation.
Still further, it is an object of the invention to provide methods which take as input, data on the concentration of each of a selected group of elements, including trace elements, and which, by way of output, quantify the concentration of each of a selected group of minerals where at least one member of the group of minerals is characterized and distinguishable by its degree of crystallinity.
It is yet a further object of this invention to provide methods for taking log data as input and providing a quantitative determination and a characterization of the clays present in the formation under investigation.
Another object of the invention is to provide a shaly sands interpretation technique which accounts for clays present in the formation.
Yet other objects of this invention are to provide methods for the analysis of depositional environment and production risks as well as improved calculation of CEC, water saturation, permeability, mean grain size and capture cross-section from the quantitative determination and characterization of minerals provided by this invention.